Accurate diagnosis of tuberculosis infection is essential for the treatment, prevention and control of this resurgent disease. Since Mycobacterium tuberculosis (MTB) is often difficult to culture from patients with active TB, and impossible to culture from healthy latently infected people, an immune-based diagnostic test indicating the presence or absence of MTB infection would be very useful for diagnosis of active TB and screening for latent MTB infection.
The only widely used test is the century-old tuberculin skin test (TST) or Mantoux test which is based on the detection of a delayed type hypersensitivity (DTH) response to an intradermal administration of a Purified Protein Derivative of the mycobacterium. This test has many drawbacks foremost amongst these is its poor specificity which results from the broad antigenic cross-reactivity of purified protein derivative (PPD), a crude mixture of over two hundred MTh proteins widely shared between MTh, M. bovis Bacillus Calmette-Guerin (BCG) and most environmental mycobacterial. Hence, false-positive results are common in people with environmental mycobacterial exposure and previous BCG vaccination. This presents a significant problem because most of the world's population is BCG-vaccinated and the confounding effect of BCG persists for up to 15 years after vaccination.
Comparative genomics has identified several genetic regions in MTB and M. bovis that are deleted in M. bovis BCG. Several regions of difference, designated RD1-RD16, between MTB or M. bovis and BCG have been identified. All represent parts of the M. bovis genome deleted during prolonged in vitro culture. RD-1 was deleted before 1921, when BCG was first disseminated internationally for use as a vaccine. RD-1 is thus absent from all vaccine strains of BCG, as well as most environmental mycobacteria, but is still present in the Mycobacterium tuberculosis complex, including all clinical isolates of MTB and M. bovis. There are nine open reading frames (ORFs) in the RD1 gene region. Early secretory antigen target-6 (ESAT-6) and culture filtrate protein 10 (CFP10) are encoded in RD-1 and have been intensively investigated in animal models and humans over the last few years. ESAT-6 and CPF10 are strong targets of the cellular immune response in animal models, tuberculosis patients and contacts and so may be used in new specific T cell-based blood tests which do not cross-react with BCG.
Cellular immune responses to gene products from RD1, RD2 and RD14 have recently been investigated in M bovis-infected and BCG-vaccinated cattle. Eight antigens were deemed to be potent T cell antigens, Rv1983, Rv1986, Rv3872, Rv3873, Rv3878, Rv3879c, Rv1979c, and Rv1769) (Cockle et al., 2002, Infect. Immun. 70:6996-7003).
However it is not possible to predict based on the antigens which are T-cell antigens in cattle which will be T-cell antigens in humans. As well as other differences in antigen processing, presentation and recognition, cattle have different MHC molecules from humans, and thus are expected to recognise different antigens.